1. Field of the Invention
The current invention relates to a conventional charge-coupled-device (CCD) camera that is optimized for the detection of living plants (green biomass) against a possibly complex (non-homogeneous) background by use of a single filter that does not require a specialized design for the CCD camera.
2. Discussion of Background Information
A number of articles in the literature teach that living green plants can be detected by use of a ratio, or a normalized ratio, of the reflectance of incident light in the near-infrared (NIR) region to reflected light in the red (R) region. One such article is entitled "Visible and Near-infrared Plant, Soil and Crop Residue Reflectivity for Weed Sensor Design," by B. B. Nitsch, K. Von Bargen, G. E. Meyer and D. A. Mortensen, which was delivered at the ASAE 1991 Summer Meeting, Paper No. 913006. The current invention is for a video sensor that can provide the information necessary to compute such ratios.
The ratio of the reflection of light from the near-infrared (NIR) and the red (R) portions of the spectrum is a reliable detector of living plants against various backgrounds of soil and crop residue. For example, this information is conveyed in the Nitsch et al. article. This ratio of NIR/R signal strength is called in this paper the "vegetative index." Alternative ratios of R and NIR may be used, one of which is the ratio of NIR-R/NIR+R, which is termed the normalized difference ratio which is described in this same article. The ratio of NIR/R may be utilized where the NIR intensity is obtained from a portion of the spectrum from approximately 700 nm to 850 nm, and the R intensity is obtained from a region around 600 nm to 650 nm. This may be used to differentiate plant growth from background soil and crop residue.
These ratios are employed in the agricultural field for the detection and identification of vegetation and growing crops, and data for these ratios are collected by three methods.
In the first method an optical fiber with a wide field of view lens is used to transmit the light to an optical beam-splitter. The beam-splitter divides the light into two channels. An NIR filter is applied to one channel and an R filter to the other channel. A ratio of the response in the two channels is used to form the NIR to R components of the intensity ratio that is selected to detect the vegetation. In a variation of this system, two separate sensors are directly employed so that the optical fiber and optical beam-splitter are not needed. Since the resolution is coarse in this version, an accurate registration of the two sensors is not necessary. This technique is described in the paper entitled "A Microprocessor Controlled Technology to Selectivity Spot Spray Weeds," by W. L. Fenton, A. F. Doss, P. G. Nash and K. R. McCIoy, Automated Agriculture for the 21st Century, Proceedings of the 1991 Symposium, 16-17 Dec., 1991, Chicago, published by ASAE.
In the second method, a spectroradiometer is employed to provide the frequency selectivity. This technique is described in the Nitsch et al. article.
In the third method, video cameras are employed to gather the data with the wavelength selectivity provided by manually inserting filters in front of the camera. This requires that the camera and the subject be stationary over the time it takes to replace one filter with the other. This technique is described in a paper entitled "The Use of Local Spectral Properties of Leaves as an Aid for Identifying Weed Seedlings in Digital Images," by e. Franz, M. R. Gebhardt and K. B. Unkesbay, ASAE 1990 International Summer Meeting, Paper No. 90-7044.
A brochure for the Sony Model XC-711/711P R.G.B. Primary Color Filter Camera discloses primary color red/green/blue pixels for CCD cameras. For this XC-711 camera, these pixels are arranged into primary color vertical stripes, as shown in FIG. 1. In some other color cameras, the pixels are arranged in a mosaic pattern, as shown in FIG. 2.
The difficulty of utilizing infrared (IR) imaging devices for the detection of cold targets is recognized in U.S. Pat. No. 5,132,802 which issued Jul. 21, 1992 to Osthues et al., and is entitled "High Contrast Image Apparatus Employing Optical Filters to Cause Each Image Pick-Up Element to have its Maximum Sensitivity in a Different Spectral Range." The apparatus of the Osthues et al. patent is described in the embodiment which is directed to the detection of military objects which may be surrounded by natural vegetation and may be camouflaged. The system of the Osthues et al. patent utilizes two image pick-ups which are selected to have two different wavelength ranges. In this patent the reflection from the vegetation is considered to be clutter. One of the wavelength ranges is selected so that the reflection characteristics of the object and of the surrounding natural vegetation are as similar as possible. The second wavelength is selected so that there is the greatest possible difference between the reflection behavior or the object to be detected and that of the vegetation.
Two separate CCD cameras are utilized in the Osthues et al. patent so that each camera receives and operates on one of the two wavelengths. The first wavelength, which is selected so that the characteristics of the object and the natural vegetation are as similar as possible, lies in the near-infrared (NIR) range. This wavelength is equal to, or greater than, the range of 0.7 .mu.m to 2 .mu.m. The second wavelength, which is utilized to distinguish the background of the object, lies between 0.2 .mu.m and 0.6 .mu.m.
Specialized CCD camera designs are shown in U.S. Pat. Nos. 4,651,001 entitled "Visible/Infrared Imaging Device with Stacked Cell Structure," issued Mar. 17, 1987 to Nozomu Harada, et al., and 4,663,656, entitled "High-Resolution CCD Imagers Using Area-Array CCD's for Sensing Spectral Components of an Optical Line Image," issued May 5, 1987 to Hammam Elabd, et al., which show infrared sensing along with visible light sensing. U.S. Pat. No. 5,350,490 entitled "Forming a Color Filter on a Semiconductor Substrate," issued Sep. 27, 1994 to Hyung S. Lee, shows how a color filter may be formed on a semiconductor substrate for CCD or bucket brigade device (BBD). The CCD device of this patent is an interline-transfer charge coupled device (IT-CCD).